Polar co-ordinate digitizers are useful for digitizing two dimensional forms because of their simplicity. The form is traced by a stylus which is connected by a cord to a relatively fixed base support at the origin axis of the polar co-ordinate system. The stylus is extensible from the base support to provide the length parameter. A cord guide through which the cord extends is pivoted at the origin and movable by the lateral pressure of the cord provides the angle parameter. Such a system is disclosed in a co-pending application, Ser. No. 286,462 filed Dec. 19, 1988, and now U.S. Pat. No. 4,922,618 (our file A-49614/JGW). Also, similar systems are disclosed in U.S. Pat. Nos. 3,832,781 and 4,500,749.
In such a system, it is always desirable to reduce complexity, and at the same time, increase accuracy. Also, it is useful for the extensible cord to be able to move through as large an angle as possible.